Electrically assisted vehicles, in which an operator of the vehicle is assisted by using a motor, are becoming prevalent. A typical example of an electrically assisted vehicle is an electrically assisted bicycle. In an electrically assisted bicycle, when a person applies a pedaling force to the pedals of the bicycle, a motor which is mounted on the bicycle adds a driving power. A similar function is beginning to be featured on vehicles other than electrically assisted bicycles, e.g., a trolley or baby stroller that moves with an applied human force. The driving power which is conferred to a vehicle by a human force is representable as “human torque”, whereas the driving power of a motor is representable as “motor torque”.
In order for the motor to be able to assist an operator of the vehicle, the power assist device needs to detect an applied human torque, and appropriately control the motor torque. As the motor outputs an appropriate motor torque, an assist is realized which is comfortable to the person. A torque sensor has conventionally been used in order for the power assist device to detect the human torque. On the other hand, there are needs for downsizing, simplification, cost reduction, etc., of the device, and thus a human torque estimation technique is needed that does not require a torque sensor.
Patent Document 1 and Patent Document 2 disclose techniques for controlling an electrically assisted bicycle without using a torque sensor.
In the bicycle with an electric motor disclosed in Patent Document 1, a transfer function of the bicycle mechanism and an inverse of the transfer function are provided in advance. The bicycle mechanism is modeled as a system that interrelates a total torque which is applied to the bicycle (including a human torque and a motor torque) and an angle of rotation of a wheel of the bicycle. The transfer function has a moment of inertia and a viscous drag as parameters defining this bicycle mechanism. By using an inverse of such a transfer function, a total torque is determined from the angle of rotation of the wheel. Thereafter, an estimated value of the human torque is determined by subtracting the motor torque from the total torque.
The electrically assisted vehicle disclosed in Patent Document 2 calculates any torque other than the motor torque as a disturbance torque, by using a disturbance observer. The disturbance torque includes a travel load torque and a human torque. This electrically assisted vehicle calculates the human torque by subtracting the travel load torque from the disturbance torque.